The present invention relates to transponder systems and more particularly to systems that contain several transponders closed to each other.
Transponder arrangements are used to identify and interrogate data in a contactless manner. Transponder systems typically include an interrogator that transmits radio frequency pulses and a plurality of transponders that receive the pulses and respond with data in the form of a modulated radio frequency carrier. However, a problem arises when multiple transponders units are present within the field of an interrogation unit and responding simultaneously as shown in FIG. 1. In this case either none or only the transponder with the strongest field strength is detected. Accordingly, what is needed is a system and method for detecting the signal of multiple transponders. The present invention addresses such a need.
The system and method in accordance with the present invention solves the problem of several transponders in the same reading field with a minimum of effort on the transponder side. This is important because in most of applications the transponder is the cost critical item. The arbitration among the transponders is solved by the fact that the reader unit sends a burst interrogation signal instead of a continuous field. The end of burst signal is detected by each transponder, and is used to launch a random delay generator. At the end of the delay, the transponder with the shortest delay sends a short signal to the reader that is interpreted as a Request To Send signal from the transponder. The reader responds with another burst that causes this transponder to send the main data and the other transponders to stop the transmission. After the first transponder has finished the data transmission, it goes to the sleep mode. The whole process is repeated until there are no more transponders left in the field. If the random delay generated in two or more transponders is the same, they start the transmission of the Request To Send signal at the same time. This collision is detected at the reader, which ignores this signal and repeats the process.